This invention relates to an electrostatic printing apparatus provided with a magnetic brush means, and more particularly to improvements on a bias voltage source for applying a bias voltage to the magnetic brush means of the electrostatic printing apparatus.
Already known is the so-called 2 rotations - 1 copy type electrostatic printing apparatus which is provided with a magnetic brush means; while a cylindrical drum makes two rotations, develops a latent electrostatically charged image or pattern produced on a photosensitive layer mounted on the outer peripheral surface of a cylindrical drum and cleans toner powder deposited on the photosensitive layer. The above-mentioned type of electrostatic printing apparatus generally includes a bias voltage source for applying a bias voltage to the magnetic brush apparatus, controls the movement of the toner particles of the magnetic brush by a potential difference produced by a bias voltage between the magnetic brush means and the photosensitive layer of the cylindrical drum, and undertakes both the development of an electrostatically charged image or pattern and the cleaning of toner particles deposited on the photosensitive layer.
With the above-mentioned type of electrostatic printing system, the problem has hitherto been pointed out that a paper sheet copied by said printing apparatus is contaminated by black band-like soils. These soils appear on that portion of the copied sheet which corresponds to the region of the photosensitive layer of the cylindrical drum contacted by the magnetic brush while the cylindrical drum stands at rest. The contamination is caused by the fact that part of the toner particles of the magnetic brush is deposited on the photosensitive layer of the drum and then undesirably transferred on to a copy sheet. It can be inferred from this fact that the aforesaid contamination does not occur while the drum is kept rotated for continuous copying, but that once the drum stands at rest, the subsequent copying is often accompanied with the contamination.
Experimental study of the above-mentioned black band contamination shows that this event arises from two causes, that is, the electrostatic deposition of part of the toner particles of the magnetic brush on a photosensitive layer of the drum and the mechanical accumulation of part of the toner particles of the magnetic brush on the photosensitive layer of the drum; the latter cause, namely contamination resulting from the mechanical attachment of toner particles to the photosensitive layer of the drum is easily eliminated during the subsequent step of development and does not appear on a copy sheet; after all, the aforesaid black band contamination is mainly caused by the former electrostatic deposition of toner particles on the photosensitive layer of the drum; the higher the concentration of toner particles in a developer mixture, the more prominent the black band contamination; and the lower the electric charge of toner particles, the less noticeable the black band contamination.
In view of the above-mentioned facts, the present inventor previously applied the under-mentioned processes to an electrostatic printing apparatus.
A toner powder used was charged with a larger amount of electric energy to provide a stronger electrostatic attractive force between the toner powder of the magnetic brush and the magnetic carrier. A photosensitive layer on the cylindrical drum was made to have a lower residual potential. These two processes indeed decreased the soiling of a copy sheet, but still proved ineffective. Therefore, difficulties accompanying an electrostatic printing apparatus have not yet been resolved.